Flexible-wing MAVsFlexible-wing MAVsDr. Peter Ifju, Bret StanfordMechanical and Aerospace EngineeringUniversity of Florida

Special ThanksSpecial ThanksStudents:

Bret StanfordRoberto AlbertaniKyu-Ho LeeSewoong JungScott EttingerMujahid AbdulrahimDon McArthurDan ClaxtonFrank BoriaMike SytsmaJos CoquytDragos ViieruBaron JohnsonMike MortonJames CliftonScott Bowman

UF Faculty:

Rick LindWarren DixonPaul HubnerWei ShyyRafi HaftkaDavid JenkinsAndy KurdilaCarl CraneWarren DixonFranklin PercivalMike Nechyba

Sponsors:

Air Force Office of Scientific ResearchAFRL at Eglin Air Force BaseUS Special Operations CommandNASA Langley Research CenterUS Geological SurveyUS Dept of Fisheries and Wildlife

James DavisYongsheng LianThomas RamboAlbert LinBrandon Evers

Design Concept: Flexible, Thin, Design Concept: Flexible, Thin, UndercamberedUndercambered Wing Wing

Undercambered wing provides better aerodynamic characteristics at Reynolds No. below 100,000.

Flexibility can be tuned for smoother flight in gusty wind conditions “adaptive washout”.

We have built wings with improved longitudinal stability.

Delayed/gentle stall has been documented

Flexible wing can be morphed efficiently.

Flexible wings can be folded for storage and deployed without assembly.

Wing configuration can be engineered to be lightweight as well as durable

Benefits of the UF DesignsBenefits of the UF Designs

MorphingMorphing

Gust AlleviationGust AlleviationStorageStorage

DurabilityDurability

Stability, high liftStability, high lift

Outline:Outline:

• Introduction•Fabrication methodologies•Flight testing•Experimental program•In-situ deformation measurements•Structural model•Fluid structure interaction models•Model validation via deformation measurements•Optimization •Conclusions and future work

Custom MAV Design SoftwareCustom MAV Design Software

• Span• Chord• Twist• Sweep• Airfoil

geometry• Virtually any

planformshape

MAVLab: rapid wing generation

CAD Model, Tool Path and Milling CAD Model, Tool Path and Milling

Finished Tooling andFinished Tooling andComposite ConstructionComposite Construction

Finished tool with layout pattern Prepreg unidirectional, woven carbon fiberand Kevlar composite construction

Composite Construction ContinuedComposite Construction Continued

Vacuumbagging

Fuselagelayup

Componentinstallation

Assembly

Finished MAV in Less Than One DayFinished MAV in Less Than One Day

• Latex rubber membrane material is applied

• Fins are attached • Off to be flight tested

International Micro Air Vehicle International Micro Air Vehicle Surveillance Competition HistorySurveillance Competition History

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

10

25

50MLB

MLBUF

UFUF

UFUF

UF

BYU

KKU

UF 4.5 in. (11.4 cm)record

Max

imum

Dim

ensi

on, c

m

15

Smallest MAV toidentify target at 600m

Year

30 cm US European MAV Competition30 cm US European MAV Competition

Three Wings were then Studied Three Wings were then Studied

[±453] [±452]

Latex Skin[02]

Rigid Batten-Reinforced BR Perimeter-Reinforced PR

• Composite wings constructed from carbon fiber composites, and latex rubber skin

• All three wings have the same nominal shape:– AR = 1.25, root chord = 130 mm, wing span = 150 mm

• Rigid wing: nominal aerodynamics• Batten-reinforced wing: adaptive washout• Perimeter-reinforced: adaptive inflation

Coefficient of Lift vs. Angle of AttackCoefficient of Lift vs. Angle of Attack

• The low aspect ratio accounts for high stall angles• After stall, the lift of the perimeter reinforced wing is greater than

The other wings before stall•The perimeter reinforced wing has higher CLmax

Moment Coefficient Moment Coefficient vsvs Coefficient of LiftCoefficient of Lift

• The perimeter reinforced wing has a higher negative slope• The rigid wing has the lowest• Static longitudinal stability of the perimeter reinforced wing is

substantially higher than the rigid case with the batten reinforcedwing intermediate

Wing Deformation Measurements UsingWing Deformation Measurements UsingVisual Image CorrelationVisual Image Correlation

• The stereo-triangulation is achieved through twin synchronized cameras (35 mm lens, 1.3 mega pixels, 5-10 ms exposure times) each looking at a different angle

• After a random speckling pattern is applied to the surface of the 3-D geometry in question, the VIC system digitally acquires the pattern, and tracks the deformation of each speckle

Synchronizedcameras

Wind tunnel

Model250 Watt lamp

Wind Tunnel VIC Tests ProcedureWind Tunnel VIC Tests Procedure

VIC Results: BR Wing OutVIC Results: BR Wing Out--OfOf--Plane Plane DisplacementsDisplacements

12° AOA, Wind Speed = 13 m/s

Wing fixed here: Non-zero displacement implies a small rigid body rotation of entire model

Primary region of deformation:battens are forced to bend upwards due to wind loading

Deformation patterns here imply that the wind load subjects the leading edge to torsion

VIC Results: PR Wing OutVIC Results: PR Wing Out--OfOf--Plane Plane DisplacementsDisplacements

12° AOA, Wind Speed = 13 m/s

Wing fixed here: Non-zero displacement implies a smallrigid body rotation of entire model

The primary region ofdeformation occurs as the membrane billows upwards dueto the aerodynamic forces

The carbon fiber perimeter exhibits substantial bending

MAV Structural ModelingMAV Structural Modeling• Accurate finite element wing modeling can provide insight into the

complicated fluid-structure interaction over a flexible MAV• In keeping with the composite nature of the wing, three different

elements are used: shells to model the carbon fiber weave (red),beams to model the battens (green), and membranes to model the latex skin (blue)

Static MAV Model ValidationStatic MAV Model Validation

• Visual image correlation is an ideal tool for finite element validation• Static model validation was conducted by hanging small weights from

the wing, and comparing numerical and experimental displacement fields

Out-of-plane displacements caused by a 7 g load at the tip of the outer left batten (MAV clamped at trailing edge)

Experimental (VIC) Numerical (FEA)

High fidelity finite elementanalysis (FEA) structural model

With nonlinear membrane properties

Navier Stokes basedcomputational fluid dynamics

(CFD) model with master/slaveperturbation techniques for remeshing

Define rigid wing geometry

Conduct CFD on rigid wing

Apply aero loads from CFD to FEA

Deformed shape analyzed by CFD

Apply new aero loads to FEA

Fluid Structure Interaction ModelFluid Structure Interaction Model

Stop when wing geometry converges

Fluid Structure Interaction Model Fluid Structure Interaction Model ConvergenceConvergence

Comparing BR Model and ExperimentComparing BR Model and Experiment

Out-of-plane displacement

Chord-wise strain

Comparing BR Model and Experiment Comparing BR Model and Experiment

Span-wise strain

Shear strain

Comparing PR Model and Experiment Comparing PR Model and Experiment

Out-of-plane displacement

Chord-wise strain

Comparing PR Model and Experiment Comparing PR Model and Experiment

Span-wise strain

Shear strain

0AOA, top

0AOA, bottom

Pressure, Streamlines and Deformation Pressure, Streamlines and Deformation Rigid Batten Perimeter

Rigid Batten Perimeter

15AOA, top

Pressure, Streamlines and Deformation Pressure, Streamlines and Deformation Rigid Batten Perimeter

Rigid Batten Perimeter

15AOA, bottom

Comparing BR Model and Experiment Comparing BR Model and Experiment

Pressure, Streamlines and Deformation Pressure, Streamlines and Deformation

PR Membrane Pretension vs. Deformation PR Membrane Pretension vs. Deformation

BR Membrane Pretension vs. Deformation BR Membrane Pretension vs. Deformation

PR Pretension vs. Performance PR Pretension vs. Performance

BR Membrane Pretension vs. Deformation BR Membrane Pretension vs. Deformation

Conclusions and Future WorkConclusions and Future Work•The design space can be greatly increased by employing flexibility

•Flight tests and wind tunnel tests have shown appreciable gains in some flight parameters with both the batten reinforced and perimeter reinforced membrane wing

•Advanced structural deformation measurement techniques provide high fidelity information that can give insight into the mechanisms that lead to enhanced flight performance

•Fluid structure interaction models can give insight into how to improve specific flight characteristics

•However no flexible wing design is the best at everything

•Topological optimization is currently being used for determiningbetter ways to reinforce the wing for specific objective functions

•Future work to validate the fluid structure interaction model byexperimentally characterizing the flow field is desired.